Warzone bosses are bosses that only appear once. Like the one Sangheili from Halo Wars that was with the Arbiter, he's a boss. Obviously there's not more than one of him. xD
Oooh didnt realise that, cool. I thought with Warden turning up in mutliplayer it meant it was like a elite zealot kind of deal, in that they're pretty unique but there is more then one.